Preparation and composition of superconducting copper oxides based on Ga-O layers

ABSTRACT

A high temperature superconducting material with the general formula GaSr 2  Ln 1-x  MxCu 2  O 7 ±w wherein Ln is selected from the group consisting of La, Ce, Pt, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Y and M is selected from the group consisting of Ca and Sr, 0.2≦x≦0.4 and w is a small fraction of one. A method of preparing this high temperature superconducting material is provided which includes heating and cooling a mixture to produce a crystalline material which is subsequently fired, ground and annealed at high pressure and temperature in oxygen to establish superconductivity.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the U.S. Department of Energy andThe University of Chicago, representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION

The present invention is directed to new compositions of matter andmethods of preparation of high temperature superconducting copperoxides. More particularly, the invention is directed to high temperaturesuperconducting materials with the general formulae of (1) GaSr₂Ln_(1-x) M_(x) Cu₂ O₇±w (M=Ca and Sr, Ln=La, Ce, Pr, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb and Y; 0.2≦x≦0.4 and w is a small fraction of one) and of(2) Ga_(1-y) Sr₂ YCu_(2+y) O₇±w (where y is about 0.65 and less).

All known high temperature superconducting copper oxides have ananisotropic structure containing two-dimensional CuO₂ layers with squareplanar, square pyramidal and octahedral coordination of the copper tooxygen. These CuO₂ layers are bounded in the third dimension bymetal-oxygen layers ("AO" hereinafter) containing large and stronglyionic metal ions (A-Ba, Sr and La-Gd), which form AO-CuO₂ -AO structuralblocks. For most superconducting compounds there is frequently more thanone CuO₂ layer, separated by metal layers, A' (A'=Ca, lanthanides andY), which are within the block, and an additional layer of mixedoxidation state cations, covalently bonded to oxygen, between theseblocks (e.g., structures based on Cu, Tl, Pb and Bi). In previousattempts, the synthesis of layered copper oxides with more ionic, fixedoxidation state, cations in this additional layer has led tononsuperconducting materials.

The importance of copper-oxygen layers in the high-temperature(T_(c) >35K) superconductors was realized in 1986 after the report byBednorz and Muller on their work in the La-Ba-Cu-O system. Theconducting planes (CuO_(4/2)) of these materials result from thehybridization of the Cu(3d) and O(2p) orbitals which form closely andsymmetrically coordinated copper and oxygen atoms in square nets. Otherfamilies of superconductors are La_(2-x) M_(x) CuO₄ (M=Ca²⁺, Sr²⁺,Ba2+), Nd_(2-x) Ce_(x) CuO₄, YBa₂ Cu₃ O_(7-x) (Tl, Bi)_(m) (Ba,Sr)₂Ca_(n-1) Cu_(n) O_(m+2n+2) (m,n=integers) , Pb₂ Sr₂ LnCu₃ O_(8+x)(Ln=lanthanides), and La_(2-x) Sr_(x) CaCu₂ O₆. All of these compoundscan be described as an intergrowth of AO rocksalt layers with ABO_(3-x)perovskite units and have the general formula (AO)_(m) (ABO_(3-x))_(n)where m and n are integers and B is copper. Although no theory on themechanism of high temperature superconductivity has gained acceptance,the observation of high-temperature superconductivity in this class oflayered materials has led to a phenomenological understanding thatsuperconductivity depends on the two-dimensional conducting planes withweak interplane coupling.

The influence of substitutions on superconductivity has been studied ingreat detail in YBa₂ Cu₃ O_(7-x). All lanthanides have been substitutedinto the eight-coordinate yttrium position. With the exception ofpraseodymium, superconductivity is preserved. In contrast, the additionof small amounts of a transition or post-transition metal onto eitherthe square-planar or square-pyramidal copper site generally resulted inthe loss of superconductivity. Incorporation of the trivalent cationsaluminium, iron, and cobalt is believed to occur on the square-planarcopper site, whereas chromium or zinc is thought to go into the planes.When large amounts of gallium or aluminium are incorporated, the newsingle-layer copper compounds LaSrCuAlo₅ and LaSrCuGaO₅ with thebrownmillerite (Ca₂ FeA10₅) structure are formed. These structuresreflect the preference for tetrahedral coordination by the groupthirteen elements. While enormous efforts have been expended in theindustry, there have been only several new systems discovered for thesetypes of structures.

It is therefore an object of the invention to provide a novelcomposition and method of manufacture of high temperaturesuperconducting ceramic.

It is another object of the invention to provide an improved compositionof high temperature ceramic superconductor based on GaSr₂ Ln_(1-x) M_(x)Cu₂ O₇±w wherein M=Ca and Sr and Ln=La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, and Y.

It is yet a further object of the invention to provide a novelcomposition of high temperature ceramic superconductor comprised ofGa_(1-y) Sr₂ YCu_(2+y) O₇±w wherein y is less than or equal to about0.65.

It is a further object of the invention to provide a novel method ofmaking a high temperature ceramic superconductor under highly oxidizingatmospheres.

It is an additional object of the invention to provide an improvedcomposition and method of making a gallium containing high temperatureceramic superconductor.

It is yet another object of the invention to provide a novel method ofoxidizing a ceramic superconductor to activate the superconducting stateof matter.

It is a further object of the invention to provide an improved method ofmodifying the crystallographic structure of a copper oxide basedsuperconductor to introduce adequate electronic charge into the Cu₂ Olayers to establish superconductivity.

It is still another object of the invention to provide a novel method ofhole doping of Cu₂ O layers of a high temperature ceramicsuperconductor.

Other objects and advantages of the invention together with the methodof manufacture and product composition will become apparent from thefollowing Detailed Description and Brief Description of the Drawingshereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the crystalline structure of LnSr₂ Cu₂ GaO₇ ;

FIG. 2A shows Squid susceptibility for dense x=o (a), 0.1 (b), 0.15 (c),0.20 (d), 0.25 (e) and 0.35 (f) samples annealed at 925° C. and FIG. 2Bshows susceptibility for an x=0.30 to 0.35 powder sample;

FIG. 3 shows the real and imaginary a.c. susceptibility for an x=0.35dense sample for various values of the a.c. field;

FIG. 4 shows exemplary resistivity as a function of temperature for fastcooled (A) and high pressure oxygen annealed (B) for x=0.35 densesamples;

FIG. 5A illustrates resistance behavior as a function of temperature forGaSr₂ Ln_(1-x) M_(x) Cu₂ O₇±w annealed at 600 atmospheres of oxygen at1100° C. and FIG. 5B shows resistance behavior as a function oftemperatures for samples prepared in a two step process;

FIG. 6 shows relative normalized resistance versus temperature forvarious annealing and cool down conditions as well as different x valuesfor the stoichimetry;

FIG. 7 illustrates the observed (+), calculated (solid line) anddifference (below) neutron diffraction pattern for LaSr₂ Cu₂ GaO₇ atroom temperature;

FIG. 8 shows susceptibility of Y₀.8 Ca₀.2 Sr₂ Cu₂ GaO₇ in the range of4°-300° K. with curve (a) the quenched sample, (b) the slow cooledsample in air, and (c) after high pressure oxygen annealing;

FIG. 9 illustrates an X-ray diffraction pattern for x=0.35 high pressureannealed sample; and

FIG. 10 illustrates a composition diagram for the Ga_(1-y) Sr₂ Y_(1-x)Ca_(x) Cu_(2+y) O₇ system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The well known high temperature superconducting copper oxide, LnBa₂ Cu₃O₇±w, contains square planar coordinated copper ions in the layer(frequently referred to as the chain region) between the AO-CuO₂-A'-CuO₂ -AO blocks. It is possible to nearly completely substitute thesquare planar copper site with cobalt. For compositions in which Ln-La,tantalum and niobium substitutions resulted in the distinct, but verysimilar, LaBa₂ TaCu₂ O₈ structure with octahedrally coordinated Nb andTa. The existence of the latter structure shows the importance of thecoordination preference of small ions in the formation of orderedlayered compounds, as opposed to mixing of these ions with copper withinone layer For AA'BCuO_(6-w) (B=transition or post-transition metal)compounds several new layered structures were discovered (e.g., singleCuO₂ layer AlSrLaCuO₅, GaSrLaCuO₅ and SnLa₂ CuO₆, and double CuO₂ layerAlSr₂ LnCu₂ O₇, FeSr₂ LnCu₂ O₇ and GaSr₂ LnCu₂ O₇).

The orthorhombic structure of GaSr₂ LnCu₂ O₇ (noncentrosymmetric spacegroup Ima2, No. 46), as determined from powder neutron andsingle-crystal X-ray diffraction data is similar to LnBa₂ Cu₃ O_(7-w)(see FIG. 1). The square-planar copper layer of LnBa₂ Cu₃ O_(7-w) isreplaced by a layer of gallium tetrahedrally coordinated to oxygen. Thelarge lanthanides and Sr are distributed over the two A and A'-cationsites, whereas the smaller lanthanides occupy only the A' site betweenthe copper planes within the double CuO₂ layer. In the instant inventionthe Ca-substituted GaSr₂ LnCu₂ O₇ compositions have been synthesizedunder high oxygen pressure such that the combined calcium and oxygendoping introduces the necessary charge to the CuO₂ layers and makes thematerial superconducting.

Polycrystalline samples of GaSr₂ Ln_(1-x) Ca_(x) Cu₂ O₇ (0≦x≦0.4) weresynthesized from stoichiometric mixture of oxides and carbonates in airat 980° C. followed by relatively rapid cooling to room temperature.Samples were fired for three weeks with frequent intermediate grindings.High pressure annealing was done for 24 hours in pure oxygen using200-500 atm. at roughly 910°-980° C. for powdered and pressed pelletsamples. Lattice parameters were determined from powder X-raydiffraction using Rietveld refinement. Susceptibility measurements wereperformed using Squid (Quantum Design Corp. MPMS) and an a.c. (LakeShore Cryotronics) susceptometers, respectively. Resistivity wasmeasured using a standard four-lead d.c. measurement. Thermogravimetricmeasurements were performed using a Cahn TG171 system.

Air cooled samples with compositions 0≦x≦0.25 for Ln-Y are single-phase.Larger doping levels led to the presence of small amounts ofunidentified impurity phases. In general, a very small contraction ofthe in-plane and an expansion of the out-of-plane lattice parameterswere observed with increasing doping. The high pressure annealed samplesshow a decreased amount of impurity phase for x≧20.25 and noticeablecontraction of the in-plane lattice parameters, indicating an increasedhole-doping of the CuO₂ planes.

High sensitivity zero field cooled Squid susceptibility measurementsusing 100 Gauss were done for both powder and pellet high pressureannealed samples. For the dense pellets annealed in pressure of about300 atmospheres at 925° C., a gradual development of a superconductingphase with an almost fixed transition temperature, T_(c) ˜20-25K, wasobserved with increased doping as shown on FIG. 2A. For powder samplesannealed in 200 atm. oxygen at 910° C., different Tc's were observedwith the highest T_(c) =73K for x=0.3 (see FIG. 2). FIG. 3 shows realand imaginary a.c. susceptibility for a x=0.35 dense sample for variousvalues of the a.c. field. Clearly, for low fields, ≦1 Gauss, the sampleshows full diamagnetic behavior, proving that for this composition alarge fraction of the sample becomes superconducting. Similar a.c. fielddependence of the measured superconducting phase fraction was observedfor the other compositions for either dense pellets or loose powders.The a.c. data also shows very good agreement for the onset T_(c)measured with the Squid.

Resistive measurements confirm superconductivity for the high pressureannealed material. Typical R vs. T data for the fast cooled (A) and highpressure oxygen annealed (B) x=0.35 dense samples is shown on FIG. 4.The resistivity changes from semiconductor-like to metallic when thesample is annealed under increasingly oxidizing conditions. Theresulting substantially linear dependence of resistivity on temperatureis the same as observed for all other high temperature superconductors.Samples annealed at the highest pressure and temperature (600 atm.oxygen at 1100° C.) show the preferred superconducting properties (seeFIG. 5A and see also FIG. 6). FIG. 5A illustrates resistance for aspecimen subjected to elevated annealing temperatures, slightly abovethe melting point of about 1100° C. (normally at atmospheric pressureT_(mp) is about 1000° C). In FIG. 5B is shown resistance behavior for asample which was synthesized in a 2 % oxygen, balance argon, atmosphereat 975° C. and then annealed at 270 atmospheres oxygen at 940° C. Thespecimen was cooled at less than about 1° C. per minute.

Superconductivity in the GaSr₂ Ln_(1-x) Ca_(x) Cu₂ O₇ system wasdetermined not to be limited to Ln=Y. Superconducting transitions up to75° K. have been observed for Ln=La, Nd, Dy and Yb, which ranges fromthe largest to the smallest lanthanide ions that form the structure. Forother Ln ions, there are no known superconducting phases containing Sr,Ca, Ln and Cu which can be prepared under the instant synthesisconditions. To verify that the presence of gallium is a necessitycondition for superconductivity, several samples have been preparedwithout Ga and processed under the same conditions as the Ga-containingmaterial. None of these samples showed any traces of eithersuperconductivity or metallic behavior.

The reduced amount of impurity phase after high pressure treatmentindicates that some additional Ca was incorporated into the compound,but the samples did not achieve the preferable full equilibrium duringthe 24-hour anneal at 910°-925° C. Therefore, although superconductingmaterial was obtained during these lower temperature anneals, higherpressures and temperatures (up to melting ˜1000° C.) favored formationof the most preferred high quality superconducting material (lowcontamination and substantially all material in the superconductingstate of matter). From thermogravimetric analysis there is a clearindication that the oxygen content increases slightly during cooling inoxygen over an extended temperature range (from about 950° C. to ˜800°C.). The cooling rates are most preferably quite slow, for example,about <1° C./min., for maximum oxygen uptake, especially for densesamples.

Without limiting the scope of the instant invention, it is believed thecompositions of interest provide a layered superconducting copper oxidewith small, fixed oxidation state cations separating the conductingAO-CuO₂ -A'-CuO₂ -AO blocks, such as, GaSr₂ Ln_(1-x) Ca_(x) Cu₂ O₇.Several similar materials, with less ionic (e.g., FeO and SnO) or moreionic (e.g., AlO, NbO and TaO) layers can also become superconductingonce doped and annealed under the previously described stronglyoxidizing conditions.

In another embodiment of the invention the composition of interestcomprises Ga_(1-y) Sr₂ Y_(1-x) Ca_(x) Cu_(2+y) O₇±w. Polycrystallinesamples were prepared for 0≦y≦0.7 from substantially stoichiometricmixture of oxides and carbonates in air at about 980° C. High pressureoxygen annealing was performed for 24 hours using about 200 to 300 atm.of pure oxygen at 940° C. followed by slow cooling at about 1°C./minute. Further details of preparation are set forth in the Examples.For this composition of invention, the optimum superconductingproperties were obtained for y values of about 0.6-0.65.

As described hereinbefore for the other system, the crystallographicstructure is the orthorhombic structure (noncentrosymmetric space groupIma2, No. 46) of the parent compound, GaSr₂ LnCu₂ O₇ which is similar toLnBa₂ Cu₃ O₇ (see FIG. 1). The solubility limits for Ca substitution onthe Y-site and Cu substitution on the Ga-site are x of about 0.25 and yof about 0.65, respectively, for air-synthesized samples. These limitsare slightly increased for high pressure annealed material. The twosubstitutions can be clearly distinguished by X-ray diffraction data.For Ca substitution, the in-plane lattice parameters (b of about 5.5Angstroms and c of about 5.4 Angstroms) decrease while the out-of-planelattice parameter (a of about 22.8 Angstroms) does not appreciablychange. For Cu substitution, the structure changes such thatcompositions beyond y of about 0.3 are tetragonal. The othersubstitution of Ca for Sr (which is up to at least about 50%) results ina considerable reduction of the "a" lattice parameter. A compositiondiagram for Ga_(1-y) Sr₂ Y_(1-x) Ca_(x) Cu_(2+y) O₇ is shown in FIG. 10.

The best superconducting properties for both of the systems describedherein are found generally near the solubility limits for bothsubstitutions. For these compositions, resistivity measurements showsuperconductivity for the high pressure annealed material (see FIG. 6).The resistance can be seen to change from semiconductor-like to metallicwhen the samples are annealed under increasingly oxidizing conditions.The almost linear dependence of resistance on temperature is the same asobserved for all other high temperature superconductors. The bulksuperconductivity (10-40%) was confirmed with d.c Squid and a.c.susceptibility measurements.

EXAMPLES

The following nonlimiting example procedures and example productsprovide illustrations of various parameters of the invention.

NEUTRON DIFFRACTION

A polycrystalline sample of LaSr_(x) Cu₂ GaO₇ was prepared by heatingstoichiometric amounts of La₂ O₃ (Aldrich, 99.99%), SrCO₂ (Aldrich99.99%) Ga₂ O₃ (Aldrich 99.99%), and CuO (Aldrich, 99.999%). The samplewas heated at 980° C. for two months with intermittent grindings. Atime-of-flight data set was collected at room temperature and ambientpressure at the Intense Pulse Neutron Source (IPNS) facility at ArgonneNational Laboratory. Approximately 8 grams of the sample was containedin a thin-walled vanadium can, and data were collected for six hours.The data from 0.568 to 2.8931 Å were used to refine the structure. Theunit cell was determined by using a nonlinear least-squares Marquetmethod to fit the peaks to an exponential rise and fall, convoluted intoa Gaussian shape, which is characteristic of the spallation neutronsource. The sample was essentially single phase, with a small LaSrCuGaO₅(<5%) impurity. A unit cell was determined from the peak positions byusing the conventional computer program TREOR and refined with theconventional program POLSQ to a 23.160 (2) Angstroms x 5.5706 (6)Angstroms x 5.4682 (8) Angsttoms orthorhombic cell. The indexationrevealed that body centering was one of the reflection conditions. Thestructure of LaSr_(x) Cu₂ GaO₇ was solved by trial and error using acell similar to YBa₂ Cu₃ O₇ as a starting model. The space group chosenwas the noncentrosymmetric space group Ima2 (No. 46), because it allowedall of the atoms to be fully ordered. The structure was refined by theRietveld method. The scattering lengths used were 8.24, 7.02, 7.718,7.288, and 5.803 fm, for the lanthanum, strontium, copper, gallium, andoxygen atoms, respectively. A total of fifty-one parameters wererefined. In the final cycle all parameters were allowed to refineundamped, including the scale factor, six peak-shape parameters, fivebackground parameters, the unit-cell parameters, positional, andisotropic thermal factors, the zero-point shift, and the diffractometerconstant, as well as the absorption and extinction parameters. Theatomic positions are shown in Table I. The final R factor was 3.29 %(4.62R_(wtd)). The observed and calculated diffraction patterns anddifference plot from 0.60 Angstroms≦ d≦2.98 Angstroms are shown in FIG.7.

                                      TABLE I                                     __________________________________________________________________________    Atomic Positions for LaSr.sub.2 Cu.sub.2 GaO.sub.7 *                               Wyckoff                                                                  atom sym  x     y    z     β, Å.sup.2                                                               occ                                            __________________________________________________________________________    La(1)                                                                              4a   0     0    0     0.45(3)                                                                           0.70(1)                                        La(2)                                                                              8c   0.1510(1)                                                                           0.9859(3)                                                                          0.9985(7)                                                                           0.36(3)                                                                           0.15(1)                                        Sr(1)                                                                              8c   0.1510(1)                                                                           0.9859(3)                                                                          0.9985(7)                                                                           0.36(3)                                                                           0.85(1)                                        Sr(2)                                                                              4a   0     0    0     0.45(3)                                                                           0.30(1)                                        Cu   8c   0.0779(1)                                                                           0.4992(3)                                                                          0.9965(7)                                                                           0.32(2)                                                                           1.00                                           Ga   4b   1/4   0.4285(3)                                                                          0.0370(7)                                                                           0.50(4)                                                                           1.00                                           0(1) 8c   0.0735(1)                                                                           0.2478(1)                                                                          0.2463(8)                                                                           0.53(4)                                                                           1.00                                           0(2) 8c   0.0700(1)                                                                           0.7513(7)                                                                          0.7492(9)                                                                           0.51(4)                                                                           1.00                                           0(3) 8c   0.1782(1)                                                                           0.5490(3)                                                                          0.9690(7)                                                                           0.85(3)                                                                           1.00                                           0(4) 4b   1/4   0.3752(5)                                                                          0.3820(8)                                                                           0.89(5)                                                                           1.00                                           __________________________________________________________________________     *Space group Ima2 (No. 46) with a = 23.1425(9) Å, b = 5.5662(2) Å     c = 5.4648(2) Å.                                                     

During the refinement it was observed that the isotopic temperaturefactors of the lanthanum and strontium sites were very different,indicating site mixing of the two atoms between the two positions. Theoccupancies were refined to 30 % strontium on the 4a site and conversely15 % lanthanum on the 8c site.

X-RAY DIFFRACTION

Single crystals of HoSr₂ Cu₂ GaO₇ were isolated as thin black platesfrom a flux with the initial composition HoSr₄ Cu₁₀ GaO_(x). The mixturewas ground thoroughly and heated to 1040° C. in air, allowed to soak forfive hours, and cooled at 6° C./hour until 700° C., followed byquenching to room temperature. The crystal chosen had the dimensions0.018 mm×0.33 mm×0.18 mm. Diffraction studies were performed on anEnraf-Nonius CAD4 diffractometer with Mo Kα (λ=0.71069 Å) radiation. Theunit cell was determined from 25 setting reflections to be a=22.696 (4)A, b=5.484 (4) A, c=5.385 (4) A. Data were collected from 2° to 90° in2θ with the conditions -h, +k, -l. An additional data set was collectedfrom 2° to 20° in 2θ with the conditions -h, +k, -l to help resolve thecentrosymetric/noncentrosymmetric ambiguity and aid in space groupdetermination. An analytical absorption correction (μ=365.91 cm⁻¹) basedon six indexed crystal faces and accurately measured distances betweenfaces was applied by using conventional TEXSAN software package. Thetransmission factors range from 0.022 to 0.510. Reflection conditions ofhkl: h+k+l=2n, Okl: k+l=2n; hOl: h, l=2n; hkO:h+k+2n:hOO:h=2n; hOO:h=2n; OkO: k+2n; and OOl: l=2n, were consistent with the centrosymmetricspace group Imma (No. 74) and the noncentrosymmetric space group Ima2(No. 46). The data were refined in Imma and Ima2 using the complete dataset consisting of all observed reflections and their Friedel opposites(790 reflections with I>3σ). Refinement of the structure in the spacegroup Imma (No. 74) with all the significant data resulted inR/Rwt_(wtd) of 13.5%/21.9%. In contrast a refinement of thenoncentrosymmetric structure and its inversion were refined to R/R_(wtd)values of the structure reported here of 6.07%/8.57% and for itsindistinguishable inversion 6.10%/8.70%. In comparing thecentrosymmetric and noncentrosymmetric solutions, the disorder in thegallium and O(4) positions required by the centric structure results inunreasonably large isotropic temperature factors for atoms O(4) andO(3). In contrast reasonable values are obtained in thenoncentrosymmetric solution without disorder. The preferred structure isnoncentrosymmetric and in the space group Ima2 No. 46).

The gallium metal position revealed a larger than expected temperaturefactor, likely indicating the presence of aluminum contamination fromthe alumina crucible used to grow the crystals. The position was refinedwith both gallium and aluminum to a more reasonable temperature factorand an aluminum content of approximately 28%, that is, the compositionHoSr₂ Cu₂ Ga₀.72 Al₀.28 O₇. Energy-dispersive X-ray analysis (EDX)studies on single crystal from the same batch confirmed aluminum to bepresent in that approximate amount. In an earlier study of theone-copper-layer brownmillerite type structure LaSrCuGaO₅, the maximumaluminum solubility at 980° C. was found to be approximately 20%. Thehigher percentage of aluminum with the double-layer structure mayreflect the higher temperature used for crystal growth. Crystallographicdata for this compound are summarized in Tables II and III. Bond lengthsand angles are presented in Table IV.

                                      TABLE II                                    __________________________________________________________________________    Summary of Crystallographic Data                                              __________________________________________________________________________    formula   HoSr.sub.2 Cu.sub.2 Ga.sub.0.72 Al.sub.0.28 O.sub.7                                           μ, cm.sup.-1                                                                           365.91                                  formula wt                                                                              637.010         temp. °C.                                                                          -120                                    cryst size                                                                              0.018 mm × 0.33 mm × 0.18 mm                                                      scan type   2θ/θ                        space group                                                                             Ima2 (No. 46)   2θ range, deg                                                                       2-90(2-20)                              lattice param. Å      indexes collected                                                                         -h, +k, -1(±h, ±k, ±1)         a         22.696(4)       R(R.sub.wtd)                                                                              0.061 (0.087)                           b         5.484(4)        no. of unique data                                                                        1678                                    c         5.385(4)        no. of unique data with                                                                   790                                                               I>3σ(I)                                       vol. Å                                                                              670.24          no. of variables                                                                          30                                      Z         4               secondary extinction                                                                      3.609 × 10.sup.-7                                           coeff                                               calcd density, g/cm.sup.3                                                               6.43                                                                __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________    Atomic Positions for HoSr.sub.2 Cu.sub.3 GaO.sub.7                                 Wyckoff                                                                  atom sym  x     y    z     β, Å.sup.2                                                               occ                                            __________________________________________________________________________    Ho   4a   0     0    0     0.27(1)                                                                           1.00                                           Sr   8c   0.8491(1)                                                                           0.0167(2)                                                                          0.085(7)                                                                            0.30(2)                                                                           1.00                                           Cu   8c   0.9265(8)                                                                           0.5008(3)                                                                          0.997(1)                                                                            0.21(2)                                                                           1.00                                           Ga   4b   1/4   0.5710(6)                                                                          0.9574(8)                                                                           0.41(6)                                                                           0.72(1)                                        A1   4b   1/4   0.5710(6)                                                                          0.9574(8)                                                                           0.41(6)                                                                           0.28(1)                                        0(1) 8c   0.9362(6)                                                                           0.762(2)                                                                           0.760(4)                                                                            0.5(2)                                                                            1.00                                           0(2) 8c   0.9366(5)                                                                           0.263(3)                                                                           0.244(5)                                                                            0.3(2)                                                                            1.00                                           0(3) 8c   0.8227(6)                                                                           0.450(2)                                                                           0.011(4)                                                                            0.8(2)                                                                            1.00                                           0(4) 4b   1/4   0.872(3)                                                                           0.105(3)                                                                            0.2(1)                                                                            1.00                                           __________________________________________________________________________     *The space group is Ima2 (No. 46) with the unit cell a = 22.696(4) Å,     b = 5.484(4) Å, c = 5.385(4) Å.                                  

                  TABLE IV                                                        ______________________________________                                        Selected Bond Angles (Degrees and Distances (Angstroms))                                LaSr.sub.2 Cu.sub.2 GaO.sub.7                                                             HoSr.sub.2 Cu.sub.2 GaO.sub.7                           ______________________________________                                        Cu-01       1.958(5)      1.94(2)                                                         1.941(5)      1.93(2)                                             Cu-02       1.967(5)      2.00(2)                                                         1.957(5)      1.88(2)                                             Cu-03       2.342(2)      2.37(1)                                             Ga-03       1.831(2) × 2                                                                          1.80(1) × 2                                   Ga-04       1.909(6)      1.92(1)                                                         1.891(4)      1.83(1)                                             Ln-01       2.570(3) × 2                                                                          2.34(2) × 2                                               2.605(3) × 2                                                                          2.47(2) × 2                                   Ln-02       2.533(3) × 2                                                                          2.38(2) × 2                                               2.536(3) × 2                                                                          2.42(2) × 2                                   Sr-01       2.678(4)      2.87(2)                                                         2.705(4)      2.74(2)                                             Sr-02       2.660(4)      2.74(2)                                                         2.671(4)      2.70(2)                                             Sr-03       2.517(2)      2.45(1)                                                         2.967(5)      2.83(2)                                                         2.654(5)      2.70(2)                                             Sr-04       2.500(2)      2.45(1)                                             01-Cu-01    89.0(1)       88.4(1)                                             01-Cu-02    90.6(2)       94.3(9)                                                         90.9(2)       86.2(9)                                                         171.7(1)      165.9(5)                                                        171.7(1)      166.3(5)                                            02-Cu-02    88.3(1)       88.1(1)                                             03-Ga-03    130.5(2)      132.6(9)                                            03-Ga-04    104.9(2) × 2                                                                          105.2(5) × 2                                              103.7(1) × 2                                                                          102.7(7) × 2                                  04-Ga-04    107.7(1)      106.4(5)                                            ______________________________________                                    

POLYCRYSTALLINE SPECIMEN ANALYSIS

Polycrystalline samples of LnSr₂ Cu₂ GaO₇ (Ln=La-Yb, Y) and "LuSr₂ Cu₂GaO₇ " were synthesized from stoichiometric mixtures of the componentoxides and carbonates as above. The components were heated to 980° C.for three weeks with frequent grinding. Lattice parameters weredetermined from an X-ray Rietveld refinement of the structure withsilicon as an internal standard and are summarized in Table V. Allsamples used for unit-cell determination were quenched to roomtemperature in air. Lattice constants for the holmium sample (see TableV) are from this study and not from the X-ray single-crystal analysis,where incorporation of aluminum during the crystal growth has slightlyaffected the lattice parameters. The lutetium compound was found not toform after prolonged heating (>three months). The resulting mixture wasfound to contain Lu₂ Cu₂ O₃, Sr₃ Ga₂ O₆, SrCuO₂ and CuGa₂ O₄.

                  TABLE V                                                         ______________________________________                                        Lattice Constants for LnSr.sub.2 Cu.sub.3 GaO.sub.7                           lanthanide (Ln)                                                                           a. A        b. A     c. A                                         ______________________________________                                        lanthanum   23.160 (2)  5.5706 (6)                                                                             5.4782 (8)                                   cerium*     22.968 (2)  5.5451 (1)                                                                             5.4400 (1)                                   praseodymium                                                                              22.955 (1)  5.5498 (1)                                                                             5.4481 (2)                                   neodymium   22.904 (1)  5.5403 (1)                                                                             5.4403 (1)                                   samarium    22.850 (1)  5.5192 (2)                                                                             5.4245 (2)                                   europium    22.839 (1)  5.5188 (1)                                                                             5.4208 (1)                                   gadolinium  22.825 (1)  5.5121 (2)                                                                             5.4167 (2)                                   terbium     22.827 (7)  5.4975 (2)                                                                             5.4057 (2)                                   dysprosium  22.807 (1)  5.4865 (3)                                                                             5.4012 (4)                                   holmium     22.818 (2)  5.4738 (6)                                                                             5.3906 (6)                                   yttrium     22.815 (1)  5.4800 (3)                                                                             5.3928 (3)                                   erbium      22.802 (1)  5.4701 (1)                                                                             5.3804 (1)                                   thulium     22.806 (2)  5.4911 (3)                                                                             5.4035 (4)                                   ytterbium   22.797 (1)  5.4606 (2)                                                                             5.3759 (2)                                   ______________________________________                                         *A small amount of cerium dioxide (C.sub.e O.sub.2) was visible in the        Xray powder diffraction pattern.                                         

MATRIX OF COMPOSITION PRODUCED

The search for optimal superconductors in Ga-Sr-RE-Ca-Cu systems weredone for a variety of compositions and synthesis conditions. The basicphase space is defined as: composition-temperature-oxygen pressure. Therange of compositions prepared include:

1) Ga-Sr₂ -Y_(1-x) -Ca₂ -Cu-O₇ system:

(a) Ga Sr₂ Y_(1-x) Ca_(x) Cu₂ O₇ with X: 0, 0.05, 0.1, 0.15, 0.20, 0.25,0.3, 0.35, 0,4, 0.5, 0.6, 0.7

(b) Ga_(1-y) Sr₂ YCu_(2+y) O₇, Y: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7

(c) Ga_(1-y) Sr₂ Y_(1-x) Ca_(x) Cu_(2+y) O₇, x=0.25 and y=0.1, 0.2, 0.3,0.4, 0.5, 0.6; y=0.6 and x=0.1, 0.2, 0.3

(d) GaSr_(2-u) Y_(1-x) Ca_(u+x) Cu₂ O₇, x=0 and u=0.1, 0.2, 0.3, 0.4,0.5, 1.0; x=0.2 and u=0.1, 0.2, 0.3; x=0.5 and u=0.5 and u=0.1, -0.1

2) GaSr.sub. 2 RE₀.75 Ca₀.25 Cu₂ O₇, RE=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, Lu

3) GaSr.sub. 2 Yb.sub. 1- x Ca_(x) Cu₂ O₇, (and the same for Dy) x=0,0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7.

4) several permutations with Ba, Sr, Ca

TYPICAL METHOD OF SYNTHESIS

The typical synthesis procedure consisted of two steps: synthesis of thematerial and anneal under high oxygen pressure.

The synthesis procedures used for GaSr.sub. 2 Y₁₋ x Ca.sub. x Cu.sub. 2O₇ system are as follows:

    __________________________________________________________________________    1st step    2nd step    cooling rate                                                                         superconducting Tc for x =                     Temp [°C.]                                                                   P(O.sub.2)                                                                          Temp [°C.]                                                                   P(O.sub.2)                                                                          deg/min                                                                              0.2                                                                              0.25                                                                             0.3                                                                             0.35                                                                             0.4                                                                              0.5 in Kelvin                    __________________________________________________________________________     980  air   --    --    2      none                                                                             none                                                                             n n  n  n                                 980  air    910  200 atm.                                                                            2      -- 25 --                                                                              -- -- --                                980  air    925  300 atm.                                                                            2      25 25 25                                                                              25 -- --                                980  air    925  300 atm.                                                                            1      -- 35 30                                                                              30 50 50                                980  air    940  300 atm.                                                                            1      -- -- --                                                                              70 -- --                                980  air    980  500 atm.                                                                            2      -- 30 35                                                                              45 45 --                                980  air    980  500 atm.                                                                            0.3    -- 35 50                                                                              50 50 --                                980  air   1050  600 atm.                                                                            1      -- -- --                                                                              50 -- --                                980  air   1100  600 atm.                                                                            1      -- -- --                                                                              -- -- 50                                980  air   1150* 600 atm.                                                                            1      -- -- --                                                                              -- -- 70                               below are also the same for Ga Sr.sub.1.9 Y.sub..6 Ca.sub..4 Cu.sub.2         O.sub.7                                                                        960  1 atm.                                                                               940  280 atm.                                                                            1      -- -- --                                                                              -- 40 --                                970  1 atm.                                                                               925  270 atm.                                                                            1      -- -- --                                                                              -- 30 --                                975  1 atm.                                                                               940  280 atm.                                                                            1      -- -- --                                                                              -- 45 --                                980  1 atm.                                                                              --    --    2      -- -- --                                                                              -- none                                                                             --                               1010* 1 atm.                                                                               940  280 atm.                                                                            1      -- -- --                                                                              -- 55 --                                985  1 atm.                                                                               940  270 atm.                                                                            1      -- -- --                                                                              -- 45 --                                990  1 atm.                                                                               940  270 atm.                                                                            1      -- -- --                                                                              -- 45 --                               1005  1 atm.                                                                               940  270 atm.                                                                            1      -- -- --                                                                              -- 50 --                                850  10.sup.-5 atm.                                                                       940  280 atm.                                                                            1      -- -- --                                                                              -- 35 --                                980  0.2 atm.                                                                             925  270   1      -- -- --                                                                              -- 70 --                                950  0.2 atm.                                                                             940  270   1      -- -- --                                                                              -- 50 --                                950  0.005 atm.                                                                           940  270   1      -- -- --                                                                              -- 70 --                                975  0.02 atm.                                                                            940  270   1      -- -- --                                                                              -- 65 --                                975  0.005 atm.-decomposed                                                    950  0.04 atm.                                                                970* 0.04 atm.                                                                925  0.05 atm.                                                                900  0.1 atm.                                                                *-- denotes temp. exceeding melting.                                          __________________________________________________________________________    And for the Ga Sr.sub.2 RE.sub.0.75 Ca.sub.0.25 Cu.sub.2 system:                       Tc[K]              Tc[K]                                                      1-step: air, 980° C., 2 deg/min                                                           0.02 atm, 975°  C.                         RE       2-step: none; 960° C., 270 atm 1 deg/min                                                  960° C., 270 atm, 1 deg/min                __________________________________________________________________________    La       no Tc    25K       45K                                               Pr       no       no        25                                                Nd       no       no        45                                                Sm       no       25        45                                                Eu       no       20        40                                                Gd       no       no        20                                                Tb       no       25        35                                                Dy       no       20        --                                                Ho       no       35        50                                                Er       no       25        60                                                Tm       no       35        70                                                Yb       no       40        --                                                Yb.sub..8 Ca.sub..2                                                                    --       --        70                                                Yb.sub..7 Ca.sub..3                                                                    --       --        75                                                Yb.sub..6 Ca.sub..4                                                                    --       --        75                                                __________________________________________________________________________     Fast cooling of a sample means the sample was removed from the furnace an     cooled on a copper plate at about ˜100 Deg/sec. Other cooling rates     are specified in the Tables.                                             

SUPERCONDUCTING CERAMIC STRUCTURE ANALYSIS

While not being limited, it is believed the LnSr₂ Cu₂ GaO₇ (Ln=La-Yb, Y)structure is best described as the replacement of the square-planarcopper in the YBa₂ Cu₃ O₇ structure with a tetrahedral gallium. Thereplacement creates a large supercell of the ideal, cubic perovskitelattice (a_(p)) where a≃6a_(p), b≃√2a_(p) and c≃√2a_(p). The Ln³⁺ andSr²⁺ "A" type cations both occupy eight-coordinate sites. The lanthanidecations preferentially occupy a 4+0+4 coordination environment betweenthe CuO_(4/2) planes. The strontium is in a more distorted 4+3+1environment. The coordination environment of the A type cations isdescribed by three numbers: the first denotes the number of oxygen atomsfrom the CuO_(4/2) layer, the second the number from AO_(4/4) layer, andthe third number from either the GaO_(4/2) layer (strontium) or theCuO_(4/2) layer (lanthanide). Because the coordination numbers are thesame for both A cations, extensive site mixing between the A cationsshould be expected for the larger lanthanides, which are similar in sizeto strontium. Less mixing should be expected for the smallerlanthanides. On the basis of the neutron diffraction study of LaSr₂ Cu₂GaO₇, lanthanum (La³⁺, 1.16 Å; Sr²⁺ 1.26 Å) was found to preferentiallyoccupy site 4a between the copper square pyramids in a 70/30 ratio. Thestrontium preferentially was on site 8c nearer the GaO_(4/2) tetrahedra.The scattering lengths for lanthanum (8.24 fm) and strontium (7.02 fm)are sufficiently different so that a refinement of possible site mixingcan be done confidently. In the X-ray single-crystal structurerefinement of HoSr₂ Cu₂ GaO₇, where ordering strongly affects theintensities, the A type cation positions were found not to bedisordered, in contrast to the lanthanum compound, but occupy distinctsites. Apparently the holmium atom (1.01 Å) is small enough so that sitemixing is not favored. This is consistent with the large (0.25 Å) sizedifference causing site specificity, although thermodynamic factorsincluding growth temperature, oxygen partial pressure, and samplehistory can also affect cation ordering.

The copper coordination for the lanthanum and holmium compounds wasfound to be square pyramidal with four short in-plane distancesaveraging 1.96 and 1.94 Å and one long apical bond of 2.34 and 2.47 Å,respectively. FIG. 1 shows the structure of the LaSr₂ Cu₂ GaO₇. Table IVcontains bond angles and distances for both compounds. Doping (p-type)studies on a number of these compounds indicate that strontium andcalcium, but not barium, can substitute for the lanthanide on the A typesite or zinc can substitute on the B site. The oxygen stoichiometry ofthese compounds has not been thoroughly investigated, but it is clearthat oxygen vacancies, if they occur, form in the plane in contrast toYBa₂ Cu₃ O₇.

MAGNETIC SUSCEPTIBILITY ANALYSIS

Susceptibility measurements were performed on a Quantum Design Corp MPMSSquid Susceptometer between 4 and 300 K. A platinum metal standard wasused for instrument calibration. The measurements were done withpolycrystalline samples encased in sealed gelatin caps. The data werecorrected for the diamagnetism of the sample holder. A 1-kG field wasused for all measurements unless otherwise noted.

The system Y-Sr-Cu-Ga-O was chosen as a candidate for superconductivitybecause of the shorter Cu-O in-plane distances and the cation orderingobserved in the structural studies (neutron and X-ray). Magneticsusceptibility experiments were performed on all samples. All sampleswere found to remain paramagnetic down to 4 K when quenched to roomtemperature in air. It was found that when doped with calcium, i,e.,Y_(1-x) Ca_(x) Sr₂ Cu₂ GaO₇ (x=0.20), a superconducting transition atabout 30 K was observed when annealed in high-pressure oxygen at about910° C. (12 hours, 200 bar) and cooled to room temperature (about 2°C./min). A plot of susceptibility versus temperature is shown in FIG. 8.An indexed powder pattern of the sample is reported in Table VI.Excellent agreement between the observed X-ray powder diffraction dataand the expected lines (see FIG. 9) based on the orthorhombic Ima2 modelindicates that the high-pressure oxygen treatment did not causedecomposition.

                  TABLE VI                                                        ______________________________________                                        Indexed X-Ray Diffraction Pattern of                                          Y.sub.0.80 Ca.sub.0.20 Sr.sub.2 Cu.sub.2 GaO.sub.7 *                          h    k     l       d.sub.Riet                                                                          I/I.sub.o.sup.c                                                                    h   k   l      d.sub.Riet                                                                         I/I.sub.o.sup.c             ______________________________________                                        2    0     0       7.74  8    10  0   2      1.740                                                                              1                           4    0     0       5.703 <1   5   3   0      1.694                                                                              2                           1    1     0       5.323 1    4   1   3      1.634                                                                              2                           3    1     0       4.442 3    14  0   0      1.629                                                                              3                           0    1     1       3.838 2    7   3   0      1.592                                                                              1                           6    0     0       3.802 <1   6   3   1      1.573                                                                              14                          2    1     1       3.638 3    12  2   0      1.561                                                                              4                           5    1     0       3.505 3    6   1   3      1.556                                                                              16                          4    1     1       3.184 5    12  0   2      1.553                                                                              4                           8    0     0       2.852 <1   14  1   1      1.499                                                                              3                           7    1     0       2.800 2    8   3   1      1.478                                                                              5                           0    2     0       2.737 28   10  2   2      1.469                                                                              2                           6    1     1       2.701 100  8   1   3      1.464                                                                              4                           0    0     2       2.692 31   5   3   2      1.434                                                                              1                           2    2     0       2.661 <1   14  2   0      1.400                                                                              4                           2    0     2       2.620 <1   14  0   2      1.394                                                                              3                           4    2     0       2.467 <1   0   4   0      1.369                                                                              3                           4    0     2       2.436 <1   2   4   0      1.359                                                                              1                           1    2     1       2.426 1    12  2   2      1.351                                                                              6                           3    2     1       2.323 1    0   0   4      1.346                                                                              3                           3    1     2       2.302 1    16  1   1      1.336                                                                              4                           8    1     1       2.289 22   14  2   2      1.242                                                                              5                           10   0     0       2.281 4    0   4   2      1.219                                                                              2                           6    2     0       2.221 4    6   3   3      1.213                                                                              4                           6    0     2       2.197 3    0   2   4      1.208                                                                              2                           5    1     2       2.135 1    18  1   1      1.203                                                                              1                           8    2     0       1.975 2    14  3   1      1.186                                                                              1                           8    0     2       1.958 2    14  1   3      1.178                                                                              2                           0    2     2       1.919 31   8   3   3      1.167                                                                              2                           12   0     0       1.901 9    6   4   2      1.161                                                                              1                           2    2     2       1.893 2    12  0   4      1.099                                                                              1                           9    2     1       1.757 1    16  1   3      1.094                                                                              1                           10   2     0       1.752 1    20  1   1      1093 3                           ______________________________________                                         *Orthorhombic; a = 22,813 (1) Å, b = 5.474 (1) Å, c = 5,384 (1)       Å. Ima2 (No. 46).                                                         .sup.b Unobserved reflections (I/Io < 1%) after 40° 2θ are       not listed.                                                                   .sup.c The numbers listed are percentages.                               

DOPING OF COMPOSITIONS

Various doping studies have been carried out utilizing thecrystallographic similarity of LnSr₂ Cu₂ GaO₇ and YBa₂ Cu₃ O₇. It wasfound that in the La_(1-x) Sr_(2+x) Cu₂ GaO₇ system the solubility ofexcess strontium in the samples was about 15 % which form thecomposition La₀.85 Sr₂.15 Cu₂ GaO₇. All compositions measured remainedsemiconductors and paramagnetic down to 4 K. In the series HoSr₂ Cu₂Ga_(1-x) Zn_(x) O₇, the samples also remained paramagnetic andsemiconducting down to 4 K. A neutron diffraction study of the x=0.15sample of the latter revealed that approximately 50% of the zinc was inthe copper planes. Similar results have been observed in other layeredcuprate systems when doped with zinc. The highest doping levelsattainable were for the Y-Sr-Cu-Ga-O system doped with calcium, i.e.,Y_(1-x) Ca_(x) Sr₂ Cu₂ GaO₇ (0≦x≦10.30). The composition Y₀.80 Ca₀.20Sr₂ Cu₂ GaO₇ remained paramagnetic upon quenching to room temperaturefrom 950° C., but slow cooling at 10° C./hour was found to greatlyreduce the susceptibility of the sample. The sample was next heatedunder high oxygen pressure and temperature (200 bar, 910° C.) andallowed to slow cool at a rate of about 2° C./minute. A small, butsignificant, fraction of the sample was found to have a superconductingtransition near 30 K. The measurement was performed in a 100-G field.Magnetization experiments (field±50,000 G) performed at 5 K revealed anH_(c1) of about 400 G and an H_(c2) of about 1800 G. We observe (seeTables V and VI) that the lattice constants of the doped, high-pressureannealed sample contract in the plane of the copper oxygen layers (b, caxes). This is expected when electrons are removed from the antibondingorbitals increasing the overlap of the in-plane copper oxygen orbitals.

CRITICAL CURRENT ANALYSIS (Ga-Sr-Y-Cu-O SYSTEM)

The critical current values were determined from magnetic measurementswithin Bean's model. The hysteresis of the magnetic moment was measuredat 5, 20 and 35 Kelvin. The magnetization hysteresis measurements forthe entire and divided sample indicate granularity, i.e., the currentloop size is limited by the grain size (1.5-1.8 μm). The calculatedJ_(c) 's values are as follow:

1) in H=O T field: J_(c) 6.7, 0.3 and 0.2×10⁵ a/cm² for T=5, 20 and 35Kelvin, respectively.

2) in H=0.5 T field: J_(c) =5.1, 0.4 and 0.2×10⁵ A/cm² for T=5, 20 and35 Kelvin respectively.

What is claimed is:
 1. A method of preparing a high temperaturesuperconducting ceramic in the system comprising Ga, Sr, Ln, Cu, and O,wherein Ln is a lanthanide element, comprising the steps of:heating to afirst temperature a mixture comprised of materials including Ga, Sr, Lnand Cu of predetermined amounts to enable obtaining the stoichiometrypresent in the ceramic system including Ga, Sr, Ln, Cu and O; coolingsaid mixture to form a starting material comprised of the combinedmaterials; and heating said starting material at a second temperature inan oxygen containing environment to produce said high temperaturesuperconducting ceramic.
 2. The method as defined in claim 1 whereinsaid second temperature is about 910° C. up to about 1100° C. for saidGa, Sr, Ln, Cu, and O ceramic system.
 3. The method as defined in claim1 wherein said oxygen is present in a pressure range of about 100-600atm.
 4. The method as defined in claim 1 where said mixture comprises atleast one oxide and at least one carbonate.
 5. The method as defined inclaim 2 wherein said superconducting ceramic further includes Ca.
 6. Themethod as defined in claim 2 wherein said crystalline material comprisesa high temperature ceramic superconductor based on GaSr₂ Ln_(1-x) M_(x)Cu₂ O₇±w wherein 0.2≦x≦0.4 and M is selected from the group consistingof Ca and Sr and Ln is a lanthanide selected from the group consistingof La, Ce Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Y.
 7. The methodas defined in claim 1, further including the step of grinding said firedcrystalline material prior to heating the material.
 8. The method asdefined in claim 1 wherein said Ga, Sr, Ln, Cu and O containing systemcomprises Ga_(1-y) Sr₂ YCu_(2+y) O with y about 0.6-0.65.
 9. The methodas defined in claim 1 further including the final step of slow coolingafter said heating step.
 10. The method as defined in claim 9 whereinsaid slow cooling step is about 2° C./minute or less.
 11. The method asdefined in claim 8 wherein said Ln is selected from the group consistingof La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Y.
 12. The methodas defined in claim 1 wherein said cooling is performed at cooling ratesless than about 5° C./minute.
 13. The method as defined in claim 1wherein said first temperature is greater than about 800° C. and lessthan about T_(mp) plus 100° C.
 14. The method as defined in claim 1wherein said oxygen containing environment consists essentially of pureoxygen at partial pressures greater than about 150 atmospheres.
 15. Themethod as defined in claim 1 wherein said heating step activates thesuperconducting state of matter of said crystalline material.
 16. Themethod as defined in claim 1, further including the step of using adopant atom to dope said crystalline material, said dopant atom selectedfrom the group consisting of Ca and O, to activate the superconductingstate of matter of said crystalline material.
 17. The method as definedin claim 1 wherein said high temperature superconducting ceramicconsists essentially of compositions between Ga₀.3 Sr₂ YCu₂.7 O₇ toGa₀.4 Sr₂ YCu₂.6 O₇.